Figure 4. The redox status (rH2) of soils with a good structure (VESS) was more oxidized than for soils with a poor structure. The colors on the points describe the soil OM concentration.

Overall, redox can be an important additional measure for soil health quantification (Table 1). It correlates with soil structure (VESS) and biological activity (CO₂burst). As it does not correlate with OM, it is a measure of the active organic matter pool, possibly similar to water soluble organic carbon (Haney et al., 2012). However, especially the pH-corrected rH₂ score correlates with soil clay content, so the interpretation should be based on soil texture. Although redox correlates with other soil health indicators, it is not a replacement for those. For example, soils with a good structure have a high redox (Figure 4), but also some poor structure soils may also have similar values. Soils with high biological activity have low redox potentials (Figure 3), but a low redox potential can also be found in a soil with moderate biological activity.

As the redox potential changes more rapidly with management than CO₂burst (Figure 2), it is a promising indicator to track management-induced changes in soil health. In this application, it could also be used to classify soils based on their Eh and pH (Figure 1). These results supported the earlier findings on the use of soil redox potential to classify soils and to follow their change due to management (Husson, 2013; Husson et al., 2016). As the earlier studies were conducted in a warm temperate climate with lower OM and clay content, similar findings in high OM and clay environments support continuing investigations into the role of redox in quantifying soil health.

A major challenge in interpreting the redox results is the integrative nature of the redox potential. It is a combined effect of all ongoing soil oxidation and reduction reactions, which defines the redox potential. It may be hypothesized that the organic matter decomposition would drive the potential in a drying-rewetting test, but in this study, soil texture was found to correlate with redox potential, suggesting that other reactions than organic matter related were also driving the potential. Further studies on the interpretation of redox in different textures and farming systems would be needed to make it an applicable tool for managing soils.

Table 1. Correlation between soil redox status (Eh and pH corrected rH2) and commonly used soil health indicators (Structure VESS; CO2burst; Organic matter; and Clay content) (Weil and Brady, 2016).